1. Field of the Invention
The invention relates to a method of steering a road vehicle having front-wheel and rear-wheel steering.
2. Description of the Prior Art
The steering behaviour of road vehicles is governed substantially by the dynamics of the yaw movement, i.e. rotational movement about the vertical axis through the centre of gravity of the vehicle. The dynamics of the yaw movement can be represented by a linear system of the second order. The movement variation depends decisively on the damping of the given values of this system, referred to hereinafter briefly as "yaw damping". The yaw damping of road vehicles in turn decreases with increasing velocity (see for example M. Mitschke, Dynamics of Motor Vehicles, Volume C, Road Holding, 2nd Edition, Springer-Verlag 1990, p. 58, FIG. 12.2).
Inadequate yaw damping at medium and higher speeds can considerably impair the roadworthiness. For example, critical situations occur when the vehicle is caught by a side wind or when the steering is disturbed by pot-holes or remnants of ice and snow. Due to his reaction time, in such cases the driver does not always succeed in stabilizing the unexpectedly occurring weakly damped yaw movement. Once this has resulted in a relatively large yaw movement the limits of the lateral tyre forces are quickly exceeded.
On the other hand, the yaw damping should not be too large because the vehicle reaction is otherwise felt to be "sluggish" by the driver.
A first generation of four-wheel steered vehicles is known still operating solely in controlled manner; a structure of such a four-wheel steered vehicle is shown in the block circuit diagram of FIG. 2. In the latter, the following symbols are employed:
.delta..sub.v (.delta..sub.h) steering angle front (rear) PA1 .delta..sub.L steering wheel command (steering wheel angle.times.steering wheel reduction) PA1 F.sub.h amplification factor for driving the rear-wheel steering PA1 .delta. side-slip angle PA1 r yaw angle PA1 a.sub.v transverse acceleration of the front axle. PA1 (a) an automatic control of the yaw motion and PA1 (b) a track guide control by the driver in which he minimizes the lateral deviation from his intended track with steering wheel movement. For this track guide control in particular the "steering transfer function" L(s) is of interest, describing the relationship between the steering command and the transverse acceleration of the front axle. PA1 (a) The yaw damping D.sub.G is independent of the vehicle velocity and PA1 (b) the numerical value of the yaw damping D.sub.G can be adjusted by the choice of the damping parameter k.sub.D in such a manner that an adjustment of the damping parameter k.sub.D does not have any influence on other properties of the steering dynamics, for example on the natural frequency of the yaw motion or on the steering transfer function from the steering wheel to the lateral acceleration of the front axle.
The mechanical relationship of the steering wheel to the steering angle .delta..sub.v of the front wheels is not changed. The rear steer angle .delta..sub.h is in the simplest case controlled proportionally to the steering angle .delta..sub.v, i.e. EQU .delta..sub.h =F.sub.h .delta..sub.v ( 1)
Usually, the amplification factor F.sub.h, the so-called "steering ratio", is made variable, in dependence upon quantities measured in the vehicle, for example travelling velocity, yaw rate, transverse acceleration. Instead of an amplification factor F.sub.h dynamic prefilters having a transfer function F.sub.h (s) are also used. The symbol s here denotes the complex variable off the Laplace transformation. In an article by E. Donges, R. Aufhammer, P. Fehrer and T. Seidenfu.beta. "Function and safety concept of active rear axle kinematics of BMW", Automobiltechnische Zeitschrift 1990, p. 580-587, the transfer function F.sub.h (s) is given in the following form: ##EQU2##
Filter parameters P.sub.r, T.sub.D and T.sub.l are calculated from the condition that a side-slip angle of zero is to be achieved. The filter parameters indicated above depend on the travelling velocity, the vehicle mass and the cornering stillnesses, which vary during operation of the vehicle. For example, the filter parameters T.sub.D and T.sub.l are proportional to the travelling velocity v. It is prior art here in automobile construction to measure the travelling velocity v and employ this for adaptation of filter parameters during operation.
A second generation of four-wheel steered vehicles employs a subordinate closed-loop control for the rear-wheel control whilst the front-wheel steering still has a conventional configuration. An example of this is the Toyota Soarer, which has been on the Japanese market since April, 1991 (cf. Hideo Inuoue, Hiroshi Harada and Yuiji Yokoja "Allradlenkung im Toyota Soarer", Congress "Four-wheel steering in automobiles", Haus der Technik, Essen, 3.-4.12.91). Here, the yaw rate r is measured with a relatively cheap vibration gyro and fed back to the rear-wheel steering; via a dynamic controller with a transfer function H.sub.h (s). Once again the symbol s here is the complex variable of the Laplace transformation. The result is a structure according to FIG. 3. The command variable w.sub.h of the subordinate control circuit is once again formed by a prefilter F.sub.h (s) adapted in accordance with the velocity.
By the feedback of the yaw rate r to the rear-wheel steering the eigenvalues of the steering dynamics can be varied and the influence of external interfering variables, for example side wind, ice on the edge of the road, road inclination, etc., can be reduced. The rear-wheel steering H.sub.h is now actuated on the basis of external disturbances as well and not only on the basis of steering wheel commands given by the driver. The compensator H.sub.h (s) is designed specifically to the vehicle, and a satisfactory compromise must be found for various travelling velocities, loads and adhesion conditions between the tyres and road surface.
The necessary compromises introduced are facilitated if a subordinate closed-loop control is provided for the front-wheel steering as well. This results in a block circuit diagram according to FIG. 1. A controller H.sub.v (s) for the front-wheel steering must also be configured specifically to the vehicle. Such a structure of the steering control system is described for example by El-Deen and A. Seirig, "Mechatronics for Cars: Integrating machines and electronics to prevent skidding on icy roads", Computers in Mechanical Engineering 1987, p.10-22.
Due to the use of subordinate control circuits the steering angle of a wheel is made up of two components. One component is effected by steering commands given by the driver and the other by external disturbances (side wind, uneven road surface, etc.). Fundamentally, a distinction can thus be made between two control objectives, i.e.
The two control objectives are however generally highly intercoupled in a control system according to FIG. 1. Both in the design of the two controllers H.sub.v (s) and H.sub.h (s) and in their adaptation to the operating conditions during travelling, any change of a controlled parameter produces both changes of the steering transfer function and changes in the yaw damping and yaw frequency.
The two control objectives can be decoupled from each other by a subordinate control of the front-wheel steering with EQU H.sub.v (s)=1s (3)
as explained in detail in U.S. patent application Ser. No. 07/753594. As a result of this decoupling the driver need only keep a mass point (imaginary at the front axle) on his intended path by lateral acceleration a.sub.v of the front axle. The yaw movement is automatically regulated and has no influence on the track guide task of the driver. The decoupling control rule thus need not be made specific to the vehicle, in contrast to all previously known controllers; on the contrary, it is governed clearly by equation (3). This decoupling is the basis of a specific variation of the yaw dynamics without simultaneously influencing the steering transfer function.
A disadvantage in all the steering control systems set forth above is that the yaw damping changes with the vehicle velocity. Since however a low damping impairs roadworthiness, it is particularly critical that the yaw damping decreases with increasing velocity.